1. Field of the Invention
This invention relates to a head positioning control system for positioning a head to stably carry out a write/read operation of data on a recording medium such as a disk in various magnetic disk drives and optical disk drives.
More particularly, the present invention relates to a head positioning control system equipped with a function of correcting the drop in a torque generated in an actuator for moving the head to a predetermined position of a recording medium in order to quickly and correctly position the head.
2. Description of the Related Art
Recently, the demand for a greater data capacity and a higher operational speed for computer systems, has been increasing, and this also holds true of auxiliary storage devices such as magnetic disk drives and optical disk drives for exchanging data with host computers. To satisfy this demand, the density of a recording medium surface of a magnetic disk drive or an optical disk drive must be increased (a track pitch of not greater than 10 .mu.m, for example).
In these magnetic and optical disk drives, a data read/write operation is carried out by controlling an actuator so as to move a head from a current track (cylinder) position on a disk to a target track (cylinder) position, and thus positioning the head to the designated target track position. In such a positioning control system, high speed positioning is a requisite without causing any vibration of the actuator, etc., even in a settling time after the movement of the head.
To enable the head positioning control system described above more easily to be understood, a head positioning system according to the prior art will be explained with reference to the accompanying drawings.
FIG. 1 is a block diagram showing a head positioning control system according to the prior art, FIGS. 2(A) and 2(B) are explanatory views useful for explaining the operation of the principal portions of FIG. 1, and FIGS. 3(A) to 3(C) are explanatory views of a VCM shown in FIG. 1. However, a positioning control system of a magnetic head in a magnetic disk drive will be explained hereby as a typical example.
Reference numeral 1 denotes a magnetic disk, 2 is a spindle motor, 3 is a magnetic head, 4 is an actuator including a voice coil motor (VCM), 5 is a power amplifier, 6 is a switch consisting of an analog switch, etc., 7 is an AGC amplifier, 8 is a regulating resistor, 9 is a position signal demodulator, 10 is a position signal selector, 11 is a velocity detector, 12 is an analog-to-digital converter (hereinafter referred to as the "A/D converter"), 13 is a controller (such as a microprocessor: MPU), 14 is a digital-to-analog converter (hereinafter referred to as the "D/A converter"), and 15 is a comparator. In FIGS. 3(A) and 3(B), reference numeral 21 denotes a rotary spindle of the actuator 4, 22 is a coil, and 23 is a magnet.
In conventional disk drives, head positioning servo control has been carried out conventionally by a head positioning control system such as the control system shown in FIG. 1, that is, a head positioning servo control circuit.
In FIG. 1, the magnetic disk 1 has the function of a recording medium which is rotated by the spindle motor 2, and the magnetic head 3 writes and reads data to and from the magnetic disk 1. Here, the magnetic heads 3 are disposed on the upper and lower surface of each of a plurality of magnetic disks 1. When servo control is effected by a servo surface servo system utilizing a servo surface, one of the surfaces of the magnetic disk 1 is used as the servo surface recording thereon the servo data.
A rotary actuator or a linear movement type actuator has been used mainly for the actuator 4, but the rotary actuator will be dealt with hereby as a typical example. This rotary actuator 4 includes a voice coil motor 40 (hereinafter referred to as "VCM") as its main constituent, and this VCM 40 moves the magnetic head 3 in a radial direction of the magnetic disk 1.
The AGC amplifier 7 effects automatic gain control for the signal read by the magnetic head 3 so as to have a constant level. The position signal demodulator 9 generates two position signals POSN and POSQ having mutual a phase difference of 90.degree. from the servo signal read by the magnetic head 3.
The regulating resistor 8 is disposed so as to regulate a feedback quantity from the position signal demodulator 9 to the AGC amplifier 7.
The A/D convertor 12 converts the two position signals POSN and POSQ to digital signals.
The position signal selector 10 selects a linear portion of the position signal POSN if the number of a track is even-numbered, and a linear portion of the position signal POSQ if the number of the track is odd-numbered, and generates a fine control signal FINS.
In the case of one magnetic disk drive having a plurality of magnetic disks 1, the positions at which the data read/write operation can be made simultaneously from these magnetic heads 3 assume a cylindrical shape. Therefore, the term "cylinder" will be used sometimes in place of the term "track".
The velocity detector 11 differentiates the position signals POSN and POSQ and detects an actual velocity as the current velocity of the magnetic head 3. The D/A convertor 14 converts a designated velocity (digital signal) output from the controller 13 to an analog signal.
The comparator 15 subtracts the actual velocity detected by the velocity detector 11 from the analog designated velocity output from the D/A convertor 14, and generates a velocity error.
The switch 6 has the function of selecting either one of coarse control for positioning control of the magnetic head 3 and high precision fine control by an analog switch, or the like, and outputs the velocity error described above in the coarse control and the fine control signal FINS in the fine control.
The power amplifier 5 drives the VCM 4 on the basis of the output of the switch 6.
The controller (e.g., MPU) 13 detects the position from the output of the A/D converter 12, generates a designated velocity in accordance with a velocity curve, outputs it to the D/A converter 14, detects the arrival of the position near the target position, and changes over the switch 6 from the coarse control to the fine control.
In the construction described above, the AGC amplifier 7, the position signal demodulator 9, the regulating resistor 8, the position signal selector 10, the velocity detector 11, the A/D converter 12, the controller 13, the D/A converter 14, the comparator 15, the switch 6 and the power amplifier 5 constitute the head positioning servo control circuit.
Next, the operation of this head positioning servo control circuit will be explained.
Receiving a seek instruction for a data read/write operation from a host apparatus, the controller 13 first changes over the switch 6 to the coarse control side.
Next, the controller 13 generates a velocity curve in accordance with the number of tracks from the current track to the target track, outputs the designated velocity to the D/A converter 14, applies the velocity error from the comparator 15 to the power amplifier 5 through the switch 6, and drives the VCM 40.
The AGC amplifier 7 controls a signal level difference between the inner periphery and the outer periphery of the magnetic disk 1 to be a constant value, by the use of the servo data read by the magnetic head 3 from the servo surface of the magnetic disk 1, and sends its output to the position signal demodulator 9.
The position signal demodulator 9 generates the two-phase position signals POSN and POSQ having a mutual phase difference of 90.degree., as shown in FIG. 2(A).
The position signals POSN and POSQ are converted to digital signals by the A/D converters 12 and are then input to the controller 13. The controller 13 detects the position of the magnetic head 3 and generates the designated velocity in accordance with this detected position.
The position signals POSN and POSQ are also input to the velocity detector 11, where the actual velocity is detected. This actual velocity is input to the comparator 15 and the comparator 15 outputs a velocity error corresponding to the difference between the designated velocity and the actual velocity.
Judging that the position reaches the vicinity of the target position by the position detection operation described above, the controller 13 changes over the switch 6 to the fine control side, and applies the fine control signal FINS output from the position signal selector 10 to the power amplifier 5.
As a result, the fine control is made for the VCM 40 and the magnetic head 3 is positioned to, and held at, the target position.
In the head positioning control described above, the position signals POSN and POSQ represent each position of the magnetic head 3. Generally, a position sensitivity varies from magnetic head 3 to head due to variance of the magnetic characteristics of each magnetic head 3. For this reason, the amplitudes of the position signals must be controlled to constant amplitudes for each apparatus.
To adjust such a position sensitivity, it has been a customary practice to regulate the regulating resistor 8 so that the amplitude of each position signal becomes constant in the state where the cylinder 0 is on-track, or to Judge the level of the position signal (e.g., 4.0 V) by repeating the seek operation to regulate the regulating resistor 8, as shown in FIG. 2(B).
Here, the VCM 40 constituting the actuator (rotary actuator) 4 of the magnetic head 3 has a structure such as shown in FIGS. 3(A) to 3(C), for example.
FIG. 3(A) shows an overall structure (plan view) of the VCM 40, FIG. 3(B) is a rear view as viewed from the back of the magnet 23, and FIG. 3(C) is a diagram showing the characteristics of the VCM 40.
As shown in FIGS. 3(A) and 3(B), the actuator 4 is allowed to rotate in directions indicated by arrows in the drawing with the rotary spindle 21 being the center.
The magnetic head 3 is disposed at the distal end of the actuator 4, and a coil 22 as the principal constituent of the VCM 40 is disposed on the opposite side of the magnetic head 3 with respect to the rotary spindle 21.
An arcuate magnet (permanent magnet) 23 is disposed round the coil 22, and the coil is disposed inside the magnetic field generated by this magnet 23.
In other words, the magnet 23 and the coil 22 together constitute the VCM. When a current is caused to flow through the coil, the coil is rotated in the directions indicated by arrows in the drawing (in both directions) between both ends a and b of the magnet 23, for example.
The voice coil is so constructed that the magnetic head 3 comes to the position of the cylinder 0 (C.sub.0) of the magnetic disk 1 when the coil 22 rotates to the end a, and to the position of the maximum cylinder n (C.sub.n) when the coil 22 rotates to the end b, for example.
In this case, the magnetic flux leaks outward at the end portion of the space defined between a pair of upper and lower magnets, and a homogeneous flux distribution is not insured. Accordingly, the flux density B tends to become lower in the vicinity of the cylinder 0 (C.sub.0) position and the maximum cylinder n (C.sub.n) position corresponding to the end portions a and b of the magnet 23, respectively, as shown in FIG. 3(C), than at the portion in the vicinity of the cylinder center.
As a result, even when a current i of the same value is caused to flow through the coil 22, a torque F=B.multidot.l.multidot.i generated by the VCM drops to the extent corresponding to the drop of the flux density B in the vicinity of both end portions (C.sub.0, C.sub.n) of the cylinder. Here, l is an effective length of the coil 22.
The head positioning control system according to the prior art described above involves the following problems.
Namely, when the head positioning control is carried out, the prior art is based on the premise that the flux density B of the magnet of the VCM is constant within a movable range of the coil.
Practically, however, the flux density B becomes lower in the velocity of the innermost and outermost cylinder regions (C.sub.0, C.sub.n) than in the vicinity of the center cylinder region, as is obvious from the characteristics of the VCM.
As a result, the torque generated by the actuator 4 drops more in the vicinity of the innermost and outermost cylinder regions than in the vicinity of the center cylinder of the head even when a current of the same value is caused to flow through the coil. If the total number of cylinders is 2,000, for example, the portions at which the torque becomes lower than a rated value are from 100 to 200 cylinders at each end portion.
Since the torque drops in this way, the actuator 4 cannot move the magnetic head 3 by the force originally expected. As a result, a longer time is necessary for the magnetic head 3 to reach the target cylinder and even after it reaches the target cylinder, the actuator 4 tends to oscillate or a longer settling time is necessary because over-shoot of acceleration of the actuator 4 is great. Furthermore, even after the position of the magnetic head 3 becomes fixed and the head starts a track following operation which permits the read/write operation, stability of the servo control system to an external impact gets deteriorated because the gain of the servo control system is less than expected.
These problems become all the more critical as the density of the surface of the recording medium such as the track surface of the magnetic disk becomes higher and higher so as to satisfy the demand for a greater recording capacity and a higher operational speed. With this increase in the density of the recording medium surface, torque fluctuation resulting from a temperature change or a change with time becomes more and more severe.
These problems can be solved, in principle, by increasing the size of the magnet and using only the central portion of such a magnet. According to this structure, however, the size of the magnetic becomes unnecessarily large and hence, the size of the magnetic drive itself also becomes large. This is quite contradictory to the recent demand for compact apparatus.
Further, as another countermeasure for addressing these problems, a method, in which absolute gain versus frequency characteristics with respect to an open loop gain in a servo control system for head positioning as illustrated in FIG. 4 are measured over the cylinders by utilizing a spectrum analyzer, has been disclosed. By means of Such a method, since a fluctuation of the gain versus frequency characteristics ranging from the central cylinder to the innermost and outermost cylinder regions is evaluated, the decrease of a torque of an actuator in the vicinity of the end positions appears to be easily corrected.
To be more specific, if the head is placed in the vicinity center of the cylinder where the flux density B is substantially constant, the gain versus frequency characteristic curve is represented as the solid line I in FIG. 4, and a zero crossing frequency f.sub.0 (Hz) where the corresponding gain value G.sub.0 1 becomes 0 dB is obtained from the solid line I. On the contrary, if the head is placed in the vicinity of either the innermost or outermost cylinder regions where the flux density B is decreased, the gain versus frequency characteristic curve is shifted toward the left direction, as shown in the dotted line II in FIG. 4. In this case, a gain value which corresponds to the above-mentioned frequency f.sub.0 is reduced to G.sub.0 2. By utilizing the correction factor K (=G.sub.0 1 /G.sub.0 2) calculated based on these gain values G.sub.0 1, G.sub.0 2, the decrease of torque in the vicinity of the innermost and outermost cylinder regions can be finally corrected.
However, in this case, it is necessary for the specified correction factors K to be calculated by measuring the respective gain versus frequency characteristic curves in various cylinder positions by means of a large-scale measuring apparatus such as a spectrum analyzer. Accordingly, the method related to FIG. 4 has a disadvantage that it takes a lot of time and labor to measure the gain versus frequency characteristics.
Furthermore, as still another countermeasure for addressing these problems, a method in which a seek time is measured by performing a seek operation in advance at each current position and each target position of the cylinder as illustrated in FIG. 5, has also been disclosed. In this method, by utilizing the fact that the seek time is increased, as the torque of an actuator is decreased due to the decrease of the flux density B in the vicinity of the end positions of the cylinder, the decrease of the torque appears to be easily corrected.
To be more specific, if the head is moved from the current position to the target position in the vicinity of the central position of the cylinder where the flux density B is substantially constant, the relatively short seek time T2 is Obtained, as shown in the curve .alpha.. On the contrary, if the head is moved in the vicinity of either end of the cylinder where the flux density B is decreased and where the torque of an actuator is not sufficiently effected in an acceleration area, the longer seek time T4 (T4&gt;T2) is obtained, as shown in the curve .beta., because the overshoot of the head occurs and continues approximately until the time T3 even after the head reaches the target position at the time T1. By utilizing the correction factor K (=T4/T2) calculated based on these kinds of seek times T2, T4, the decrease of torque in the vicinity of the end positions of the cylinder can be also corrected.
In such a method, a large-scale measuring apparatus such as a spectrum analyzer is unnecessary. However, the seek time must be measured in advance by every combination of each current position and each target position of the cylinder. Consequently, as the number of cylinders is increased due to the increase of track density of the disk, the time and labor for previously measuring the seek time is likely to be increased, and therefore the method related to FIG. 5 is not practicable.